1. Technical Field
The present invention relates to a method of producing a piston ring in which particles for the prevention of piston shoulder wear are embedded. The present invention further relates to a piston ring which has been produced using the method according to the invention.
2. Related Art
Piston rings in an internal combustion engine seal the gap existing between the piston head and the cylinder wall in relation to the combustion chamber. During the upward and downward movement of the piston, the piston ring on the one hand slides with its outer peripheral surface in constant resilient abutment with the cylinder wall, on the other hand the piston ring slides, due to the tilting movements of the piston, in an oscillating manner in its piston ring groove, wherein its shoulders alternately abut the upper or lower groove shoulder of the piston ring groove. Depending on the material used, a greater or lesser degree of wear occurs in the sliding partners running against one another in each case, said wear leading to fissures, scoring and ultimately to destruction of the engine in the case of dry running. In order to improve the sliding and wear performance of piston rings in relation to the cylinder wall, said rings were provided with coatings of different materials on their peripheral surface.
During dynamic engine running, piston rings are exposed to axial movement in the piston groove on account of gas, frictional and inertia forces. They are subject to continuous sliding wear due to their constant contact with the cylinder bore. This is manifested both as abrasive wear of the piston ring surface or its coating and also as the partial transfer of material from the cylinder running surface to the piston running surface and vice versa. This causes abrasive wear on the shoulders of the piston ring, which severely affects the functional performance of the piston ring. This shoulder wear has a direct effect on the engine's emission performance.
Cast iron materials or cast iron alloys are mainly used to produce piston rings. Piston rings, particularly compression rings, are subject to an increasing load in high-performance engines, including compression peak pressure, combustion temperature, EGR and lubricating film reduction, which have a crucial effect on the functional properties of said engines, such as wear, burn mark resistance, microwelding and corrosion-resistance. State-of-the-art cast iron materials exhibit a high fracture risk, however, which means that when the previous materials are used, ring fractures frequently occur. Increased mechanical-dynamic loads lead to shorter service lives among piston rings. There is also severe wear and corrosion on the running surface and the shoulder. Due to the greater mechanical and dynamic stresses on piston rings, ever more engine manufacturers are calling for piston rings and cylinder liners made from high-grade steel. In this case, ferrous materials with a carbon content below 2.08% by weight are referred to as steel. If the carbon content is higher, they are referred to as cast iron. Steel materials have better strength and toughness properties compared with cast iron, as there is no interference due to free graphite in the basic structure.
Today's piston ring materials based on cast iron or steel constitute a homogeneous material, which itself is insufficiently resistant to shoulder wear. In order to reduce shoulder wear in piston rings in engines, wear protection layers are applied to the piston ring shoulders. Piston rings with particle-reinforced hard-chrome coatings exhibit significantly improved abrasion resistance, for example, compared with uncoated or nitrated rings, but also compared with piston rings with conventional hard-chrome and plasma-injected layers on a molybdenum base. However, these coatings have also come close to the limits of their performance capability, due to the increasing pressure and temperature parameters in modern combustion engines. There was therefore a need for new coatings with even lower wear and greater adhesion resistance than had previously been the case. In order to satisfy this requirement, composite powders were applied to piston ring surfaces by means of thermal spraying, said powders containing a ceramic phase in a metal matrix. In this way, it is possible to combine the good tribological properties of ceramics with the good mechanical properties of metals. A tough, ductile fusion of the hard and in some cases brittle ceramic particles in the metallic matrix is guaranteed. The ceramic particles can then assume tribological functions with suitable exposure on the surface of the piston ring, while the metal matrix absorbs the mechanical loads and reduces stresses where required via deformations.
The production of piston rings coated in this manner is however costly, as a coating stage must be added on to the production of the piston ring. The problem addressed by the present invention is therefore that of creating a method of producing piston rings, which are provided with ceramic particles on their shoulders, without a coating step being required to introduce these ceramic particles.